1. Field of the Invention
The present invention relates generally to interface circuits and more particularly, it relates to a line receiver circuit having an input hysteresis characteristic which is compensated for both temperature changes and variations in TTL compatible supply voltage.
2. Description of the Prior Art
It is generally known that all electronic circuits have difficulties when noise is encountered. While digital circuits typically have a higher tolerance to noise than analog circuity, they are not completely immune from it. Thus, it has come to be extremely important in many applications for logic circuits to be capable of operating reliably in a noisy environment. In an effort to increase the noise immunity of logic circuits, there have been attempted in the prior art of providing hysteresis in the threshold voltage so that a high threshold voltage is generated in a line receiver circuit when the input logic signal is in the low level and a low threshold voltage is generated when the input logic signal is in the high level. This has the effect of increasing the ability of a circuit to withstand short, noise voltage pulses that appear at the input to the logic circuit without causing a change of states as its output.
Such a circuit of the prior art which provides an input hysteresis characteristic has been manufactured and sold by Advanced Micro Devices, Inc., under the part designation of Am 26S12/12A. However, this circuit is compensated for temperature only and does not compensate for variations in the supply voltage.
Another prior art circuit having an input hysteresis characteristic which is compensated both for temperature and supply voltage variations has been manufactured and sold by Texas Instruments, Inc. However, this circuit is designed to be operated with two supply voltages of +12 volts and -12 volts, which is not a TTL compatible supply voltage.
The present invention represents an improvement over these prior art circuits. The hysteresis line receiver of the instant invention provides both a high threshold voltage and a low threshold voltage which can be switched between the two threshold voltages. Further, the threshold voltages are maintained in a stable condition over temperature changes and variations in a TTL compatible supply voltage.